222 A BOOK OF INSECTS 



It has sometimes been stated that this flower can only be 

 fertilised by the humming-bird hawk-moth {Macroglossa 

 stellatarum), no other European insect having a proboscis 

 long enough to reach the extremity of the spur ; but this 

 is incorrect. Though common in the British Islands, the 

 humming-bird hawk-moth is somewhat uncertain in its 

 appearance, and one may keep watch in a garden for days 

 together without seeing one of these insects. But the 

 ubiquitous humble-bees frequently visit Nasturtiums during 

 the summer months, and there can be no doubt that they 

 are largely responsible for the abundance of seed which the 

 plants produce. For the Nasturtium is self-sterile — i.e. 

 incapable of effecting the pollination of its own stigma. 

 " When it first opens " (writes Lord Avebury) " the anthers 

 are unripe, the pistil is short and immature. Soon, how- 

 ever, one of the anthers ripens, opens, and turns up, so 

 as to stand directly in front of the opening to the tube ; a 

 humble-bee, therefore, or other insect of similar size, 

 visiting the flower for the sake of its honey, could not 

 fail to rub some of the pollen off on her breast. Shortly 

 afterwards a second stamen ripens, and assumes the same 

 position, with the same result, and the rest gradually 

 follow. In flowers which I have watched, this process 

 occupies from three to seven days, by which time the 

 stamens have all come to maturity, after which the anthers 

 drop off, and the filaments turn down so as to be well out 

 of the way. It is now the turn of the pistil, which in the 

 meantime has elongated, and assumes the position which 

 the stamens had successively occupied; the result of 

 which is that a bee which had previously visited a younger 

 flower and dusted her breast with pollen could not fail to 

 deposit some of t lie pollen on the stigma." 



The reader will perceive that any increase in the length 

 of the floral tube must tend to restrict the number of 



